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Swedish and British researchers have used the European Space Agencys Cluster spacecraft to unveil the mysteries of the "black aurora", a strange electrical phenomenon that generates dark, empty regions adjacent to the visible Northern and Southern Lights.

The new results, to be announced today at a meeting of the American Geophysical Union in San Francisco, confirm that these patches represent a kind of "anti-aurora", where conditions are the exact opposite of those in the normal aurora.

The data show that the black aurorae represent holes in the ionosphere, the part of the upper atmosphere where aurorae are created. Here, negatively charged electrons are being accelerated upwards into space inside regions known as "positively charged electric potential structures".

This is the opposite process to that which creates visible aurorae, where electrons spiral down from space into the atmosphere within similar, negatively charged, structures.

"The black aurora isn't actually an aurora at all; its a lack of auroral activity in a region where electrons are expelled from the ionosphere," explained Professor Göran Marklund of the Alfvén Laboratory in Sweden. "The black aurora is visible to the naked eye only if it is embedded in a region of diffuse (faint) aurora."

"Now, with the aid of the four Cluster spacecraft, we have been able to study for the first time the complex physical processes that create these auroral holes," he said. "Cluster has allowed us to discover how the huge vertical structures associated with the black aurora form, how long they last and how they vary with altitude."

Clusters String of Pearls.

The first Cluster observations took place on the morning of 14 January 2001, when the quartet were cruising through the magnetosphere. From an altitude of more than 21,600 km, the four spacecraft were able to study the conditions associated with the aurora far below. Aligned like a string of pearls, the quartet swept from south to north across the northern aurora at 100-second time intervals.

In a matter of minutes, the first three spacecraft (using the PEACE instrument) detected a threefold increase (from about 700 to 2000 electronvolts) in the energy of the upward electrons. At the same time, the EFW experiment measured the increasing electric field that was accelerating the electrons, while the FGM magnetometer measured the electric current. Shortly after, when the last spacecraft reached the same position, the electric field had vanished and no evidence of a structure could be seen.

Similar results were obtained from a crossing of the southern hemisphere auroral zone on 14 February. Once again, the electric field increased steadily in strength, but the electric current stayed constant as the quartet sped through the upper reaches of the U-shaped structure.

"The data show that the potential structures that create the black aurora extend to altitudes greater than 20,000 km and that they grow in size and intensify over time scales of a few minutes," said Professor Marklund. "It is as if the cosmic battery was getting stronger and stronger and then suddenly stopped working after about 3-4 minutes."

"This period of growth is comparable to the time it takes to suck the electrons from parts of the ionosphere," said Marklund.

"This is the first time that we have been able to follow the evolution of these structures as they accelerate the electrons away from the auroral ionosphere," he added.

"Understanding the development and growth of these dynamic structures associated with the aurora is a major goal of the Cluster mission, and something which cannot be solved by single satellite measurements," he concluded.

The results will be published in the journal Nature on 13 December 2001.

Notes for Editors.

The name "black aurora" was first coined about 40 years ago, when visual observers first noticed unusually dark spaces in the diffuse Northern Lights. Diffuse auroras are pale, nondescript auroral displays about as bright as the Milky Way.

Three major types of black aurora have been recognised: black curls that corkscrew across the sky; black rings, which look like dark smoke rings; and black patches, which resemble giant black blobs in a sea of faint aurora.

The ionosphere is a sparse layer of atmosphere filled with billions of electrons and ions (positively charged atomic particles). It is located at an altitude of between 60 km and 600 km above the Earth. This is the home of the aurorae  the Northern and Southern Lights.

The visible aurora is generated by electrons swirling down the Earths magnetic field lines and colliding with the upper atmosphere.

The four Cluster spacecraft were built and launched in 2000 by the European Space Agency (ESA). Cluster is part of an international programme, which includes ESA's SOHO and Ulysses satellites, to find out about how the Sun and Earth interact. The Cluster quartet will perform in-situ measurements of the Earth's magnetosphere using, for the first time, four identical spacecraft. These measurements will provide an accurate three-dimensional picture of the behaviour of the magnetosphere.